The invention relates generally to a fiber optic gas sensor, and more particularly to a nano-structured trampoline integrated fiber Bragg grating sensing device for gas molecular weight detection.
Fiber optic sensors are widely used in the oil and gas industry for monitoring down-hole and turbomachinery parameters such as pressure, temperature, gas composition, hydrocarbon flow and seismic status. Monitoring of such machine operation parameters or/and operation conditions may provide real-time control and optimization for production output and enable condition-based maintenance for prolonging a machine's lifetime. In general, simultaneously measuring both temperature and pressure can be practically done with conventional pressure gauges and thermocouples. Gas composition analysis is commonly based on gas chromatography (GC), micro gas chromatography (MGC), or a mass spectroscopy. More specifically, gas composition analysis is typically based on either laser spectroscopy by analyzing molecular vibration modes, laser and photo-acoustic absorption at specific wavelengths, or on gas chromatography by separating the components of a gas mixture primarily based on boiling point (or vapor pressure) differences.
In many instances, online gas composition and molecular mass analyses are required either for efficiency control or for machine operation control. However, using GC or laser absorption instrument online for monitoring an industrial process, such as analyzing a gas mixture, including multiple gas compositions, is challenging due to time-consuming and complicated instrument field calibration. In one case, the gas analysis requires knowing each composition variation such as in gas fuel quality monitoring; in other case, the gas analysis is for measuring effective gas molecular weight, the ratio Cp/Cv of the constant-pressure specific heat over the constant-volume specific heat, and gas heating value variation, such as in gas charge compressor or compressor train for ethylene production process monitoring. Although, gas characteristics, such as gas molecular weight, are desired to be known, they are not easily determinable and a standard design with nominal value is used for compressor efficiency prediction and control.
Fiber Bragg Grating (FBG) sensors are well known in the art for monitoring the various parameters and machine operations previously noted. More specifically, FBGs are more commonly used to measure temperature, pressure, flow, vibration, strain, and displacement, etc. One advantage is that FBGs can be wavelength multiplexed along one fiber, making them attractive for multi-point measurements or multi-measured measurements. A lesser known application is to use a FBG sensor for gas composition, purity, cleanness, and molecular mass analyses. The technical barrier for using FBG for measuring gas is due mainly to the chemical inactive nature of the silicon dioxide fiber material.
As disclosed in U.S. Pat. No. 7,489,835, U.S. Patent No. 7,400,789 and U.S. Pat. No. 7,489,835 fiber Bragg gratings, integrated with a gas sensitive sensing material layer, have shown measurable wavelength shifts induced by fiber cladding refractive index variation of the sensing layer that interacts with gas molecular. However, these sensors are designed for identifying a specific gas and, more preferred to be used for specific single-component gas analysis. The gas induced sensing layer refractive index change is calibrated with gas concentration. However, it is difficult to analyze a gas mixture that has multi-component, where the interference from different gases could greatly degrade the sensor's accuracy and reliability. Another challenge is that the sensing layer could be contaminated by real industrial environment where the oil mists, particle deposits, and polymerization may exist.
For measuring effective gas molecular weight and the specific heat ratio (k=Cp/Cv) variation, such as from gas charge compressor or compressor train for ethylene production process, it is not necessary to measure each component variation from a multi-component gas mixture. By considering the critical nature in measuring these gas characteristics for efficiency control and optimization, it will be helpful to have an online gas sensor for real-time operation for gas molecular weight and specific heat ratio measurements. Such a gas sensor should be insensitive to multi-component gas interference, oil mists, water vapor, tar deposit, and heavy hydrocarbon polymerization etc.
Accordingly, it is desirable to provide a low-cost gas sensing device and sensing instrumentation for the evaluation of gas characteristics to provide online accurate data for broad industrial gas applications such as measuring gas purity, gas quality, and gas cleanness. More specifically, there is a need to develop a new kind of gas sensor for determining gas characteristics, and in particular for measurement of gas molecular characteristics as online system for chemical petrochemical, refinery, and power generation industrial process control and optimization.